1. Field of the Invention
The present invention relates to a pre-transfer charging device for image processing apparatus in xerography applications. In particular the invention relates to construction of a pre-transfer charging device shield provided adjacent an imaging unit that is mounted in a separable frame of image processing apparatus in a xerographic image reproducing machine.
2. Description of Related Art
Xerographic image processing apparatus in photocopiers and facsimile devices will generally be provided with an imaging unit from which a toner-developed image retained on a photoconductive body is transferred onto a copy sheet, whereupon the sheet is conveyed to a fixing unit which heat-fixes the toner image formed superficially on the sheet. Typically, the imaging unit comprises a photoconductor drum as the photoconductive body, and during a xerographic operation, an electrostatic latent image corresponding to an original is formed on the drum cylindrical surface. A drum charging device, an electrostatic image developing device, an image transferring device, a sheet separation device and a cleaning device are located in turn around the photoconductor drum (which assembly, less the image transferring device, will be referred to hereinafter by the term imaging unit).
Further, a copy sheet conveyance path along which fresh copy sheets are supplied to the imaging unit is provided upstream, in terms of the flow of copy sheets through the image processing apparatus, of the imaging unit. A sheet conveyance terminal guide for guiding the sheets into a position between the imaging unit and the image transferring device is provided at an end of the copy sheet conveyance path along the photoconductor drum.
In the example of xerographic photocopiers, a general construction of the machine body main frame will comprise a plurality of separate frames joined so as to be movable relative to one another in order to facilitate maintenance operations. In a clamshell type photocopier, for example, the main frame is divided into an upper frame openable on a lower frame, with the image transferring device, sheet separation device, sheet conveyance guide, and associated components, located in the lower frame, and the imaging unit located in the upper frame.
Moreover, in order to improve the efficiency with which the image transferring device transfers the toner image on the photoconductor drum onto a supplied copy sheet, a pre-transfer charging device can be provided immediately adjacent the drum between the developing device and the sheet conveyance guide. The pre-transfer charging device, which for example can be a discharger wire surrounded by a conductive shield, or a discharging lamp, causes charge under the toner-developed image superficially retained on the photoconductor drum to disappear, thereby improving the efficiency of the succeeding transfer of the toner image onto a sheet by the image transferring device.
In a clamshell type copier equipped as described above, the positioning of the pre-transfer charging device relative to the photoconductor drum must accordingly remain accurate. Therefore, it is preferable to fit the pre-transfer charging device in the upper frame, in which the photoconductor drum is mounted. This configuration, however, inconveniences maintenance in cleaning, since the pre-transfer charging device must be drawn out in total from the upper frame in order to clean its interior.